penis size, like all other traits, would not come exclusively from the father. when DNA combines the nucleotides do not stop and say "hey, lets do the penis section now, remember, only transcribe from the dad's chromosones." having a similar penis to your father does not actually mean anything other than that you have a similar size. For this reason it is also it is also unreasonable for people to suppose that because a certain family member was either poorly or well-endowed that they ought to be the same. you don't hear about people freaking out because their dad or brother had blue eyes and they didn't. odd as it may sound your mother is carring two sets of penis size characteristics from both of her parents, as your father is also carrying another set besies his displayed phenotype. i would also guess it is a combination of genes that produce final penis size (hormone release, ligament length, sub-epidermal fat layer, circulation, ect).